The present invention relates to adaptive equalizers which can be utilized in data transmission systems to compensate for the amplitude and delay distortion that is present in many communication channels.
Adaptive equalizers are necessary for accurate reception of high-speed data signals transmitted over band-limited channels with unknown transmission characteristics. The equalizer is generally in the form of a transversal filter in which a sampled signal comprised of samples of an analog data signal is multiplied by respective tap coefficients. The resulting products are added together to generate an equalizer output which is then used to form an estimate of the transmitted data. In addition, an error signal is formed equal to the difference between the equalizer output and the estimate of the transmitted data. This error signal is used to update the tap coefficient values in such a way as to minimize both the noise and distortion--primarily intersymbol interference--introduced by the channel. The most commonly used error-directed tap coefficient updating algorithm is the so-called least-mean-squared (LMS) algorithm, which adjusts the tap coefficients so as to minimize the mean-squared error (MSE)--the average of the value of the square of the error signal.
Many high-speed data receivers incorporate a synchronous, or baud, equalizer in which the analog data signal is sampled at a rate equal to the symbol rate. It is, however, possible to use a so-called fractionally spaced equalizer in which the analog data signal is sampled at a rate higher than the symbol rate. Data decisions, i.e., quantizations of the equalizer outputs, are still made at the symbol rate. However, the fact that equalization is carried out using a finer sampling interval provides the fractionally spaced equalizer with significant advantages over its more conventional cousin. Most notable among these is insensitivity to channel delay distortion.
There is, however, at least one significant problem unique to the fractionally spaced equalizer. In a synchronous equalizer one set of tap coefficients is clearly optimum, i.e., provides the smallest mean-squared error. By contrast, in the fractionally spaced equalizer, many sets of tap coefficient values provide approximately the same mean-squared error. As a consequence of this property, the presence of small biases in the tap coefficient updating processing--such as arithmetic biases associated with signal value round off--can cause at least some of the tap coefficient values to slowly `drift` to ever-larger levels, even though the mean-squared error remains at, or close to, its minimum value. The registers used to store the tap coefficients or other signals generated during normal equalizer operation may eventually overflow, causing severe degradation, or total collapse, of the system response.
The prior art, as exemplified in U.S. Pat. Nos. 4,237,554 and 4,376,308, teaches two alternative approaches to solving the problem of tap coefficient drift. Specifically, U.S. Pat. No. 4,237,554, issued Dec. 2, 1980 to Gitlin et al., teaches that tap coefficient drift can be constrained, i.e. reduced to acceptable levels, by use of a "tap leakage algorithm" in which a constant magnitude term is factored into the tap coefficient update algorithm in order to control the magnitude of the tap coefficient values. However this solution requires additional processing power in the equalizer in order to implement the operations which comprise the tap-leakage algorithm. Its use may not even be possible in some situations, e.g. in cases where it is desirable to implement the equalizer using an existing `off-the-shelf` VLSI component, whose internal algorithm cannot be modified. Alternatively, U.S. Pat. No. 4,376,308, issued Mar. 8, 1983 to B. McNair, teaches that tap coefficient drift can also be constrained by the injection of an additional signal, derived from the analog data signal into the "out-of-band" frequency region of a fractionally spaced equalizer. Specifically, a composite signal is formed from the analog data signal and the additional signal, whereby equalization of the composite signal constrains the tap coefficient drift of the equalizer. Although providing an alternative approach, this technique requires a fairly sharp filter to constrain the additional signal to the out-of-band region. A sharp filter is essential because any energy appearing in the fractionally spaced equalizer's "in-band" frequency region, from this additional signal, will increase the mean-squared error. As a result, due to the necessity of using sharp filters, considerable additional processing power or hardware is also required.